A speaker enclosure using a passive radiator usually contains an "active loudspeaker", and a passiveradiator. The active loudspeaker is a normal driver, and the passive radiator is of similar construction, but without a voice coil and magnet assembly. It is not attached to a voice coil or wired to an electrical circuit or power amplifier. In the same way as a ported loudspeaker, a passive radiator system uses the sound otherwise trapped in the enclosure to excite a resonance that makes it easier for the speaker system to create the deepest pitches. The passive radiator resonates at a frequency determined by its mass and the springiness of the air in the enclosure. It is tuned to the specific enclosure by varying its mass. Internal air pressure produced by movements of the active driver cone moves the passive radiator cone. This resonance simultaneously reduces the amount that the woofer has to move.
Design considerations
Passive radiators are used instead of a reflex port for several reasons. In small volume enclosures tuned to low frequencies, the length of the required port becomes very large. They are also used to reduce or eliminate the objectionable noises of port turbulence and compressive flow caused by high velocity airflow in small ports. In addition, ports have pipe resonances that can produce undesired effects on frequency response. Passive radiators are tuned by mass variations, changing the way their compliance interacts with motion of the air in the box. The weight of the cone of the passive radiator should be approximately equivalent to the mass of the air that would have filled the port which might have been used for that design. The frequency response of a passive radiator will be similar to that of a ported cabinet, but the systemlow frequency roll-off will be slightly steeper due to a notch in frequency response caused by the Vap of the passive radiator. This notch occurs at the PR's free-air resonant frequency and causes slightly poorer transient response. Despite this, perhaps due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers are only slightly more complex to design and are generally more expensive as compared to standard bass reflex enclosures.
Applications
Passive radiators are used in Bluetooth speakers, home stereo speakers, subwoofer cabinets and car audio speaker systems, particularly in cases where there is not enough space for a port or vent system. While most studio monitor speakers are either ported bass reflex designs, or closed-back without a vent or passive radiator, Mackie's HR824 and HR624 monitor speakers have a passive radiator installed on the rear of the cabinet. Focal also sells a studio monitor with a passive radiator called the SM9.
